1. FIELD OF THE INVENTION
The present invention relates to an improved process for the production of ethylene oxide and relates more particularly to the preparation of ethylene oxide by reacting ethylene with oxygen on silver-based catalysts.
2. DESCRIPTION OF THE PRIOR ART
As is known, in the direct oxidation of ethylene to ethylene oxide a gaseous mixture containing relatively small quantities of ethylene and oxygen is conveyed over a silver-based oxidation catalyst.
Catalytic oxidation is usually brought about at temperatures of approximately 150.degree. to 400.degree. C and at pressures varying from atmospheric pressure to approximately 30 atmospheres. The catalyst contains silver which is usually deposited on a granular support such as mullite, silica, alumina, silicon carbide, magnesium oxide or the like. The catalyst can also contain smaller quantities of other metals such as platinum, gold, palladium, barium and calcium. The latter substances are called activators of the ethylene oxidation reaction.
As is known, in the aforementioned process of oxidation only a part of the ethylene is converted to ethylene oxide and more or less charge quantities of total oxidation products, such as water and carbon dioxide, are formed.
Consequently, the gaseous reaction products are treated in order to separate by-products other than ethylene oxide, after which the residual gas stream is recycled to the oxidation reactor.
There are various prior-art methods of obtaining reaction gases rich in ethylene oxide and reducing as much as possible the amounts of by-products in the ethylene oxidation reaction. A high content of ethylene oxide in the reaction gases is desirable, both for reasons of productivity and because it is easier to recover the gases. A low quantity of by-products indicates both a low consumption of valuable products and a simplification of that part of the apparatus which is used for eliminating the by-products.
Various methods of obtaining these desirable results have been tried in the prior art, e.g., the use of improved catalysts or the supply of special substances together with the reacting gases, the special substances either modulating the oxidation reaction or varying the method of oxidizing ethylene.
Attempts, for example, have been made to influence the catalyst, either by carefully choosing the support, or by varying the manner in which silver is precipitated on the support. As is known, the properties of the carrier and of the deposited silver have an important effect on the activity and selectivity of the catalyst in the oxidation reaction of ethylene to ethylene oxide.
It is also known in the prior art to use special substances having a moderating effect on the ethylene oxidation reaction. In general organic compounds containing halogen or nitrogen are used as moderators.
These compounds are used in very small quantities and prevent the organic compounds in the reaction mixture from being completely oxidized.
In other known methods, the oxidation of ethylene to ethylene oxide is made more productive by partially replacing the inert gas in the gaseous supply mixture by paraffins (usually methane) containing a small number of carbon atoms. Methane, which restricts the explosive zone, enables the gaseous supply mixture to contain higher concentrations of reagents.
Finally, the conditions under which ethylene is oxidized, more particularly the temperature control, are also important. As is known, relatively large quantities of heat are generated during the ethylene oxidation reaction and tend to increase the temperature both of the gas and of the catalyst. Consequently, unless the heat of the reaction is rapidly removed, hot points develop on the catalyst, with a consequent further increase in the heat developed owing to the increased speed of the reaction. The final effort is a reduction of the yield of ethylene oxide through complete combustion to carbon dioxide and water.
The usual method of avoiding hot points is to limit the maximum temperature of the oxidation reaction so as to control the speed at which heat is developed in the reactor. The temperature can also be controlled by using reactors consisting of a number of tubes filled with catalyst and surrounded by a cooling medium.
The aforementioned precautions can improve the method of oxidizing ethylene to ethylene oxide but have not produced completely satisfactory results. The known methods cannot increase productivity, i.e., the concentration of ethylene oxide in the reaction gases, beyond a relatively modest limit without undesirably increasing the by-products, i.e., without causing an reduction of the selectivity of the reaction.
This is due at least partly to the fact that it has not been possible to effectively influence the course of the temperature in the reaction zone, more particularly in those methods using a single oxidation reactor, and to the fact that it has not been possible to make complete use of the properties of the aforementioned moderators in the ethylene oxidation reaction.